Interactions of positively identified human natural killer (NK) cells known to be killing appropriate target cells have been studied in serial thin sections with the electron microscope. NK cells have a characteristic type of granule containing bundles of tubules in crystalline arrays. The tubule bundles resemble structures formed by microtubule protein (tubulin) under some conditions. Within an hour of binding to a target cell several types of granule within the killer cell, including tubule-containing granules appear to fuse and the heterogeneous contents of the fused granules are secreted by exocytosis into the space between the killer and target cell. The secreted material is mostly in the form of membrane-like structures some of which have ring-shaped profiles on their surfaces. The morphology suggests that tubulin associated with membrane vesicles is released from the killer cell. The secreted products cling to and possibly fuse with the target cell membrane.